This invention relates in general to color-reproducing television cathode ray tubes and more particularly to means for suspending a novel color selection electrode or "shadow mask" in proper adjacent parallelism to the faceplates of such tubes.
The conventional color selection electrodes or "shadow mask" comprises a dished apertured mask which is welded to a rigid frame that imparts the necessary rigidity for the dished section. This type of mask-frame assembly is mounted in close adjacency to the faceplate of the cathode ray tube by a suspension system comprising three or four leaf springs. The springs are welded to the frame at selected points around the periphery. Because of the weight of the mask, it is necessary that the springs be relatively stiff; this stiffness results in the application of a load directed radially inwardly of up to four or five pounds to the mask-frame assembly. The distal ends of the springs are apertured to engage studs which project inwardly from the rearwardly extending flange of the tube faceplate. It is necessary that the mask-frame assembly be capable of being demounted several times, and remounted with exact precision in relation to the faceplate; this demounting and mounting is required in the manufacturing process. Demounting is accomplished by depressing the springs to disengage the studs, usually by automatic machinery. In remounting, the shadow mask and the faceplate are brought into propinquity whereby the springs are caused to re-engage the studs.
The type of mask described; that is, one having a heavy, rigid frame which in turn provides the necessary rigidity to the dished, apertured mask, has significant disadvantages despite its proved commercial viability. The disadvantages include costliness, excessive mass, heavy weight, and the tendency to distort upon heating under the impact of the cathode ray tube electron beams. As a result of this distortion, it has been necessary to design elaborate and costly shadow mask suspension systems wherein bimetallic components provide the necessary "Q-spacing" to compensate for such heating. Q-spacing is defined as the spacing between the shadow mask and the phosphor screen. The Q-spacing must be accurately maintained in order for proper registration of mask apertures with the associated phosphor stripes (or dots).
The problem inherent in a rigid mask design as described has led to the invention of a low cost, light-weight, non-self-rigid, torsionally flexible shadow mask, preferably of one-piece, frameless construction. A shadow mask of this type is disclosed in U.S. Pat. No. 4,100,451 to Palac, of common ownership herewith, wherein an approximately rectangular, flangeless curved faceplate supports on a concave inner surface thereof in a central region a phosphor screen comprising patterns of red-, blue-, and green-emissive phosphor triads. The low mass, approximately rectangular, non-self-rigid, torsionally flexible shadow mask has a central portion with a pattern of electrontransmissive apertures in registry with the patterns of phosphor triads. The mask has a rim portion providing substantial rigidity with respect to axes normal to the sides thereof, while providing for flexure of the mask with respect to its diagonals. The mask suspension system establishes a predetermined position of the mask relative to, and at a predetermined spacing from, the inner surface of the faceplate. The system includes four suspension means for mechanically coupling the mask directly to the corner portions of the faceplate. The suspension means are located on at each corner of the mask to permit the mask to flex about its diagonals and conform to the contour of the faceplate despite any twist-wise deformation thereof. By this means, the predetermined spacing between the mask and the faceplate inner surface is maintained.
The present invention is an improvement of a suspension system for a low cost, light weight, non-self-rigid, torsionally flexible mask, which is compatible with the standard flanged face panel design. The faceplate is used to impart the necessary rigidity to the mask. The novel suspension system furnishes a mechanically rigid link between the faceplate and the mask, and yet permits the mask to be conveniently and repeatably demounted and precisely remounted in the tube.
There are a number of basic requirements imposed upon any mask suspension system. The first requirement, a general one, is that the suspension system must be able to adapt to normal "out of plane" errors of the four panel-mounted studs and to twist-wise deformations of the faceplate with which it is mated, and which may occur during the manufacturing process. The mask must be capable of flexing or twisting about its diagonals in much the same way that a faceplate is apt to twist-wise deform in contour during tube fabrication, and its suspension system must provide for such adaptation.
Second, and of equal significance--with respect to any given faceplate, since the mask is non-self-rigid, the suspension system for the mask must effectively transfer the rigidity of the faceplate to the mask.
Third, the suspension system must precisely fix and hold a predetermined spatial position of the mask as a whole relative to the faceplate against translational or rotational displacement in spite of any thermal expansion or contraction of the mask, frictional restraint during demounting and remounting of the mask, mechanical shocks, and force of gravity.
Fourth, it is desirable that any thermally induced movement of any part of the mask or of any mask suspension element during tube operation be radial rather than tangential, since radial errors can be compensated by adjusting in the beam deflection characteristic, whereas tangential errors cannot be.
Fifth, it is desirable that the system permit the mask to be conveniently and quickly demounted and remounted, preferably automatically, since in conventional factory faceplate screening practices the mask is mounted on or demounted from the faceplate many times.
A sixth general requirement is that the mask suspension system should carry a low manufacture cost. A different type of shadow mask and suspension system thereof is disclosed in the patent to Fyler--U.S. Pat. No. 2,961,560. This patent shows a frameless shadow mask supported at a multiplicity of spaced peripheral points directly on projections from the concave screen-bearing surface of the tube faceplate. By this approach, it would appear that the rigidity of the faceplate is used to impart rigidity to the mask, thus eliminating the necessity for the mask to also be rigid. The Fyler approach would appear to suffer, however, (1) from an intolerable difficulty and inconvenience in the demounting and remounting of the shadow mask in the tube, an operation performed many times on conventional faceplate screening practices; (2) a difficulty in seating and reseating the mask uniformly on the multiplicity of support elements provided on the faceplate; (3) uncontrollability of the spatial position of the mask corners, and thereby a loss of color purity in the corners of the displayed images; (4) a probable shifting of the geometrical center of the mask upon thermal expansion and contraction thereof due to the non-equalized, frictional retention of the mask in the Fyler mask mounting system; (5) difficulty in achieving a commercially satisfactory Q-compensation of the mask if such is necessary, and (6), a relatively high cost of system manufacture and assembly.
As will be pointed out in more detail hereinafter, this invention involves the provision of a shadow mask suspension system comprising four suspension devices, one at each corner of the tube faceplate with each device including an axially extending, cantilevered leaf spring. It has been found that numerous additional specific requirements are imposed upon such a system, devolving in part from the corner location of the suspension devices, and in part from the use of a cantilevered-type spring as an element of the device.
A seventh specific requirement is as follows. In order to achieve the afore-discussed fixing of the spatial position of the mask, in the context of a four-corner cantilevered spring suspension system as described, it has been discovered that at least three of the springs must be reasonably stiff in the plane of the spring. If the mask suspension springs are not sufficiently stiff in the plane thereof; i.e., in the tangential direction as mounted, and preferably (though not necessarily) in torsion also, the mask will not always return to its bogey position (nominal assigned position) after having received a mechanical shock or after having been demounted and remounted. This fact is due largely to the mass of the mask and to friction at the points of engagement of the mask-mounted and envelope-mounted components of the mask suspension devices.
An eighth important requirement of the mask suspension system is that it provide a relatively constant and relatively low-value radial spring loading on the mask, without the imposition of any significant moment tending to twist or deform the mask. Yet the mask must be supported against mechanical shocks which, e.g., may apply 45 G's or more to the mask. This requirement is especially important in a suspension system designed, as the present system is, especially for use with a lightweight, non-self-rigid mask capable of being distorted or deformed by an excessive loading or by a moment loading thereof.
A ninth (specific) requirement is that, in order that the suspension device not occupy a large area in the corner of the faceplate, which would require the provision of a larger-than-desired faceplate (and associated funnel), the deflection of the leaf spring to effect engagement or disengagement of the mask from the faceplate must be quite small. Further, the spring must not be so large as to require the provision of an intolerably great amount of space in the corner of the faceplate to accommodate the spring.
Tenth, the spring must be of a thickness, for certain embodiments of the invention, to be suitable for welding to a supporting structure. Further, the spring must not be over-stressed during demounting or remounting of the mask, and during thermal cycling of the tube during tube fabrication.
A prior art patent to Haas--U.S. Pat. No. 2,922,063--discloses a suspension system for a shadow mask which appears in some respects similar and in other respects very different from the suspension system of the present invention. Haas discloses a shadow mask having a lightweight frame to which is attached a perforated color-selection mask. The mask is suspended adjacent to concave inner surface of a faceplate of the type having a rearwardly extending flange.
A suspension system is shown for suspending the mask which comprises four suspension devices located on the major and minor axes of the faceplate, each suspension device including a relatively wide leaf spring which is attached to the faceplate adjacent the seal land and extending forwardly to the mask frame. The springs are said to be "thin, flat metallic strips so that they are stiff in a lateral dimension but flexible in a direction perpendicular to the major flat face" (column 3, lines 48-51).
The Haas system is considered to have a number of major shortcomings which have perhaps been responsible for its apparent failure to have achieved commercial use. Each leaf spring is apparently so flexible out of its own plane as to require that it be either screwed to a sealed-in flange (FIG. 4) or held on a faceplate-embedded stud by means of a special spring clip (55 in FIG. 5). In either arrangement, it is possible that any thermal expansion of the mask of frame would result in a moment being applied to the mask-frame assembly which would distort the mask and produce color impurity in the displayed images.
The Haas system would be further unsuited for use in the present system for failure to meet the aforedescribed first, fifth, and ninth requirements, and perhaps others.
Yet another prior art approach is expounded by U.S. Pat. Nos. 3,450,920; 3,497,746; 3,529,199; 3,548,235; British No. 1,278,633; British No. 1,278,634; British No. 1,278,635 and British No. 1,172,334. In these systems, a shadow mask having a deep-drawn integral mask skirt, either with or without a frame, is mounted adjacent to the concave inner screen-bearing surface of a faceplate of the type having a rearwardly extending flange. Numerous ways are shown by which such a mask may be suspended on the faceplate flange by means of mask-mounted elements which are received in recesses formed integrally in the faceplate flange. In certain embodiments, the recesses are suggested for location on the faceplate flange sides. In other embodiments it is suggested that the recesses be located in the faceplate flange corners. The basic approach described in these patents would be totally incapable of meeting a number of the basic requirements imposed on a system of the type with which this invention is involved, described above. As a practical matter, it is impossible to consistently and repeatedly achieve the necessary accuracy in mask-to-faceplate registration in any mask suspension system in which large and unpredictable friction forces are produced. U.S. Pat. No. 3,529,199 to Duistermaat et al also discloses a more conventional stud-spring suspension system (FIG. 2) but this too is deemed to be ineffective in meeting the needs of a system of the type with which this invention is concerned.
U.S. Pat. No. 3,999,098 to Dougherty discloses an embodiment of an invention wherein a non-self-rigid shadow mask is shown as providing for mounting in conjunction with a flanged faceplate. The system provides for four corner-located mask suspension devices which are attached to studs embedded one in each corner of the rearward flange of the faceplate. The shadow mask is characterized by having a integral skirt which flares outwardly to shield the screen from stray and overscanned electrons. The skirt and an integrally formed channel and edge lip enhance the stiffness of the mask with respect to its major and minor axis, while permitting the mask to flex with respect to its diagonals. The system includes suspension springs which extend forwardly in an orientation which would appear to preclude facile mounting and demounting of the mask.
U.S. Pat. No. 1,189,403 to Phillips Electronics and Associated Industries Limited, discloses a color television picture tube wherein a shadow mask is provided with a transversely protruding peripheral flange-like wall extending away from the associated faceplate. The mask, which is four-corner-mounted by means of four support springs, is reputedly secured in the tube without the use of a supporting frame. One end of each spring is welded to a wall extending from the mask. At the opposite end of each spring is attached a pin which enters an associated hole (or alternatively, a groove) located in the faceplate flange, and formed by drilling. The support springs are designed to lie at an angle of approximately one-half the deflection angle of the tube; this choice of angle is said to cause the mask to move toward the screen when the mask is heated under electron bombardment. Drawbacks to the system include the relatively short length of the springs. The stiffness of the springs and the resulting pressure on the mask could overflex the mask upon engagement and disengagement with the faceplate. Also, the relatively acute angle of the spring could result in translational misalignment upon vibration, and disengagement of the springs when the tube is face-dropped. Further, the mask flanges appear to restrict access to the springs, which must be retracted towards the mask during engagement and disengagement with the faceplate. Welding springs of such design directly to a relatively tiny area of the relatively thin metal of the mask, as indicated by FIG. 1 of the disclosure, may result in distortion of the mask due to induced stresses.
U.S. Pat. No. 3,943,399 to Sedivy, assigned to the assignee of the present invention, depicts suspension systems for detachably supporting a rectangular non-self-rigid, torsionally flexible shadow mask on the envelope of a color cathode ray tube. The suspension system comprises four corner-located mounting springs. In the preferred embodiment illustrated, the envelope-mounted means and the mask-mounted means mate with a snap-in, self-guiding, self-locating engagement means such that the mask may be mounted on the envelope by a push-click insertion operation. The suspension springs comprise loops extending between the mask frame and supporting studs and they would seem to occupy an untoward amount of space. Stability of such a mask assembly appears questionable.
U.S. Pat. No. 3,986,072 Adamski discloses a four-corner system for suspending a shadow mask having an integral skirt in proper spaced adjacency to a flangeless faceplate. In one embodiment, the suspension system includes a modified stud having a pair of legs which are embedded one in each corner of the rearwardly extending flange of a flanged faceplate (FIG. 17). The forward-facing suspension springs, which are attached to a bracket at one end and mated with studs at the other, are relatively short and wide, and hence relatively stiff. The integral skirt is cut away at the corners to provide clearance for the stud which is embedded in an extension of the screen-bearing inner surface of the faceplate. This embodiment, while feasible, lacks practicality for the present application in that the forewardly disposed suspension springs are in a relatively inconvenient location and orientation for easy and facile mounting and demounting of the mask.